The invention relates to a method for manufacturing grids to be incorporated in nuclear reactor fuel assemblies. It is particularly suitable for the manufacture of grids for assemblies whose framework comprises end pieces connected by guide tubes imprisoned in some of the cells of grids spaced apart evenly along the guide tubes, the other cells of the grids supporting the fuel rods of the assembly.
Among such grids, reference will particularly be made to those whose major faces have a square shape and which comprise two sets of inner plates disposed in two orthogonal directions and interlocked together so as to define the elementary cells through which the fuel rods and the guide tubes pass. The inner plates are connected at their peripheries, to plates forming a belt. These component parts are secured together at their intersections to provide cohesion of the grids.
Grids are also known, particularly for undermoderated reactor assemblies, which have a hexagonal cross-section and which comprise three sets of intersecting plates. Although particularly advantageous in the case of grids whose major faces have a square shape and whose cells are distributed above the nodes of the square lattice, the invention also applies to grids whose major faces have a hexagonal shape.
The component parts of the grids have generally been secured together by manual brazing, which is a long and tedious operation and which, in addition, does not guarantee fully satisfactory reproducibility.
Different methods and devices have however been proposed for welding the component parts together in a more or less automatic way.
The invention relates to a method of the kind described in French Patent No. 2,522,560, in which the grid whose component parts are to be secured together is placed in a mechanical shaping and holding frame having passages for access to the points to be welded on the two major faces and on the sides of the grid; the frame containing the grid is gripped by a device, placed in a gas-tight chamber containing an inert gas atmosphere; weldings is carried out on one face at a time, using a laser beam orthogonal to the axis of rotation of the device, delivered by a laser generator placed outside the chamber and penetrating thereinto through a transparent window. The operation is then repeated on the other faces after manually repositioning the grid.
By welding in an inert gas protective atmosphere, oxidation of the welds is avoided, which makes it possible, in particular, to apply the method not only to grids made of a high strength alloy, such as INCONEL, but also to grids whose components are made of a zirconium-based alloy.
From a reading of French Patent No. 2,522,560, it seems that the protective atmosphere is obtained by scavenging, which involves considerable inert gas losses. The laser generator is held fixed during welding of a face, except for the welds carried out on the belt, for which the beam is deflected by means of a set of mirrors. The variety of accurate movements of the grids inside the chamber does not guarantee sufficient accuracy to be compatible with the very small size of the focus spot of the laser beam. In addition, the chamber must have a very large volume.
FR-A-2532216 and EP-A-0102252 also describe a method for welding using a fixed laser generator. The laser beam is directed and focussed in an open chamber, subjected to argon scavenging, in which a grid is driven with orthogonal displacement movements. This method requires very considerable gas flows, collection of the gases polluted by the laser shots is difficult and controlling leaks in the open chamber requires very reduced clearances, which are difficult to obtain.
U.S. Pat. No. 4,710,606 describes a welding robot in which a laser beam must be moved along five degrees of freedom by mirrors, which implies a great complexity of the mechanism and accumulation of tolerances.